The present invention relates to a light-emitting diode (LED) lamp and more particularly relates to an LED white light source.
An LED is a semiconductor device that can produce an emission in a brilliant color highly efficiently in spite of its very small size. And the emission produced by an LED has an excellent monochromatic peak. To produce white light by diffusing and combining the emissions of multiple LEDs, a color mixing process is needed. For example, three LEDs, each producing an emission at a wavelength in the red, green or blue range of the visible spectrum (which will be herein called red, green and blue LEDs, respectively), should be placed closely to each other. However, each of these LEDs has an excellent monochromatic peak. Accordingly, the white light produced by mixing these colors with each other is often uneven. That is to say, where the emissions in the three primary colors cannot be combined together in a desired manner, the resultant white light will be uneven. To eliminate this problem of color unevenness, a technique of producing white light by using a blue LED and a yellow emitting phosphor in combination was developed as disclosed in Japanese Laid-Open Publication No. 10-242513, for example.
According to the technique disclosed in this publication, white light is produced by combining the emission produced by the blue LED with the luminescence that is exhibited by the yellow emitting phosphor when photoexcited by the emission of the blue LED. In this case, white light is obtained using only one LED. Accordingly, the problem of color unevenness, typically observed where multiple different types of LEDs closely disposed are used to produce white light, is avoidable.
In the prior art, LEDs have been mainly applied to display devices. For that reason, the use of multiple LEDs in combination as a white light source (or general illumination lamp) has not yet been researched and developed sufficiently. Where LEDs are used for a display device, just the light emitted spontaneously from the LEDs should have its properties optimized. However, where LEDs are used in combination as a lamp, the white light, with which an object will be illuminated, should have its color rendering performance adjusted as well. And in the current state of the art, no one has ever succeeded in developing an LED lamp with optimized color rendering performance.
It is true that the LED disclosed in the above-identified publication can contribute to producing white light. However, we found that the following problems arise where the LED and yellow emitting phosphor are used in combination as an LED lamp. Specifically, the known LED lamp produces white light by utilizing the emission of the blue LED and the luminescence of the yellow emitting phosphor. Thus, the spectral distribution of the light produced by the LED lamp lacks the emission spectrum for red spectral components covering a wavelength range of 600 nm or more. In that case, if the blue LED with the phosphor is used as an illuminator or backlight, the red spectral components of the object illuminated cannot be reproduced sufficiently. In addition, since the white light is short of the red spectral components, it is difficult to make an LED white light source with a relatively low correlated color temperature. We found it hard for the known white light LED lamp to have a general color rendering index Ra exceeding about 85 even where the light produced by the lamp has such a color as requiring relatively high correlated color temperature (i.e., even if the white light may short of red spectral components). If the white light is appraised by a special color rendering index R9, indicating how brilliant the color red illuminated by the light source look to the human eye, then the red color reproducibility of the white light is obviously poor, e.g., the special color rendering index R9 is as low as about 50.
To compensate for lack of the red emission spectrum covering the wavelength range of 600 nm or more, we also modeled a modified LED lamp that further included a red emitting phosphor in addition to the blue LED and yellow emitting phosphor as disclosed in the publication identified above. However, such a lamp results in very low energy conversion efficiency, because the red spectral components are made up for by getting the red emitting phosphor photoexcited by the emission of the blue LED. That is to say, producing a red emission with the blue emission means a long conversion wavelength, thus considerably decreasing the energy conversion efficiency in accordance with the Stokes"" law. As a result, the LED lamp will have its luminous efficacy decreased too much. Thus, it is not practical to add a red emitting phosphor to the blue LED and yellow emitting phosphor.
It is therefore an object of the present invention to provide an LED lamp with good color reproducibility and high luminous efficacy.
An LED lamp according to the present invention includes blue and red LEDs and a phosphor. The blue LED produces an emission at a wavelength falling within a blue wavelength range. The red LED produces an emission at a wavelength falling within a red wavelength range. The phosphor is photoexcited by the emission of the blue LED to exhibit a luminescence having an emission spectrum in an intermediate wavelength range between the blue and red wavelength ranges.
In one embodiment of the present invention, the red LED may produce the emission at a peak wavelength of 600 nm or more.
In another embodiment of the present invention, the blue LED may produce the emission at a peak wavelength of between 450 and 470 nm. The red LED may produce the emission at a peak wavelength of between 610 and 630 nm. And the phosphor may exhibit the luminescence at a peak wavelength of between 520 and 560 nm.
In still another embodiment, if the LED lamp has a correlated color temperature of 5000 K or more and if color rendering performance of the lamp is appraised using a reconstituted daylight source as a standard source, the blue LED may produce the emission at a peak wavelength of between 450 and 460 nm, the red LED may produces the emission at a peak wavelength of 600 nm or more and the phosphor may exhibit the luminescence at a peak wavelength of between 520 and 560 nm.
In an alternative embodiment, if the LED lamp has a correlated color temperature of less than 5000 K and if color rendering performance of the lamp is appraised using a black-body source as a standard source, the red LED may produce the emission at a peak wavelength of between 615 and 650 nm and the phosphor may exhibit the luminescence at a peak wavelength of between 545 and 560 nm.
In yet another embodiment, the inventive LED lamp may further include means for controlling an intensity of the emission produced by the red LED.
In this particular embodiment, the control means may be a variable resistor.
More specifically, the blue LED preferably produces the emission at a peak wavelength of between 455 and 465 nm, the red LED preferably produces the emission at a peak wavelength of between 620 and 630 nm, and the phosphor preferably exhibits the luminescence at a peak wavelength of between 540 and 550 nm.
In yet another embodiment, the phosphor may be a yellow emitting phosphor that exhibits a yellow luminescence when photoexcited by the emission of the blue LED.
In that case, the yellow emitting phosphor is preferably either a YAG phosphor or a phosphor doped with Mn as a luminescence center.
In an alternative embodiment, the phosphor may be a green emitting phosphor that exhibits a green luminescence when photoexcited by the emission of the blue LED.
Then, the green emitting phosphor is preferably either a YAG phosphor or a phosphor doped with at least one element selected from the group consisting of Tb, Ce, Eu and Mn as a luminescence center.
In yet another embodiment, the blue and red LEDs and the phosphor may be integrated together within a single envelope.
In that case, a site at which the blue LED produces the emission thereof and a site at which the red LED produces the emission thereof may be both located in a single chip.
In yet another embodiment, an LED light source, which includes the blue LED and the phosphor, and another LED light source, including the red LED, may be separately disposed to make a cluster.
An inventive lamp unit includes: at least one LED lamp according to the present invention; and a power supply for supplying power to the lamp.
In one embodiment of the present invention, the lamp unit may further include a reflector for reflecting light produced from the lamp(s).
An LED lamp according to the present invention includes a red LED and can compensate for lack of the red emission spectrum covering a wavelength range of 600 nm or more without decreasing the luminous efficacy of the lamp. Thus, the present invention provides an LED lamp that can produce white light with good color reproducibility and high luminous efficacy.
Also, where the peak wavelengths of the blue and red LEDs and phosphor are between 450 and 470 nm, between 610 and 630 nm, and between 520 and 560 nm, respectively, an LED lamp with good color reproducibility is realized. Furthermore, where the inventive LED lamp additionally includes means for controlling the luminous intensity of the red LED, the luminous intensity of the red LED is easily controllable. Accordingly, the inventive LED lamp can arbitrarily change the color of the light produced.